There is known a precipitation strengthened Ni-based superalloy in which fine precipitates composed of an intermetallic compound are dispersed in an Ni matrix. Such an alloy has been widely used as parts that require mechanical strength under high temperature environment, for example, parts for a gas turbine or a steam turbine. As a representative alloy, there may be mentioned a γ′-precipitation strengthened Ni-based superalloy which contains Ti and Al forming intermetallic compounds with Ni and in which γ′-phase of the intermetallic compound is finely dispersed in a γ-phase that is an Ni matrix. However, in such an alloy, when the γ′ phase is excessively precipitated, hot workability decreases and crystal grains cannot be fined by forging, so that good mechanical strength cannot be obtained.
For example, Patent Document 1 discloses a method for producing an Ni-based superalloy material in which γ′ grains are coarsened by overaging to secure hot workability and fining of crystal grains is attained at a forging step, in a γ′-precipitation strengthened Ni-based superalloy containing an increased amount of the γ′-phase as compared with an alloy that is referred to as Waspaloy. In this method, an alloy lump is heated to a temperature higher than the solvus temperature Ts to form a solid solution of the γ′-phase and then, it is slowly cooled to allow the γ′-phase to precipitate and grow to form an overaged structure. Subsequently, forging and rotary forging are further performed at a temperature lower than Ts, thereby obtaining fine crystal grains of ASTM 12 or more. In this method, the solvus temperature is set to be from 1,110 to 1,121.1° C., which is higher than that of a common same-type alloy species. This is because the forging temperature can be raised and forging resistance can be lowered even when the forging is performed at a temperature of Ts or lower without forming a solid solution of the γ′ grains.
Moreover, Patent Document 2 discloses a method for producing a precipitation strengthened Ni-based superalloy material that may contain a large amount of the γ′-phase. In this method, an ingot is held at a temperature of the solvus temperature Ts or lower to allow a part of the γ′-phase to form solid solution, and then slowly cooled, thereby transforming the γ′-grains into coarse grains having an average particle size of 1.5 μm or more by overaging, thereby securing hot workability. Subsequently, the alloy structure is fined by extrusion processing while promoting recrystallization. It is said that voids generated on this occasion are eliminated by subsequent HIP treatment.
In addition, Patent Document 3 discloses a method for producing an Ni-based superalloy material in which a hot-processed material is subjected to slow cooling overaging and forging at a predetermined temperature of the solvus temperature Ts or lower to obtain a disconformable γ′ phase which does not have continuity to the crystal lattice of the γ-phase that is a matrix and does not have a large influence on mechanical strength, thereby securing hot workability. After sizing by forging, a solution treatment is performed to transfer the disconformable γ′ phase into a solid solution again and a conformable γ′-phase is then precipitated by performing an aging treatment.    Patent Document 1: JP-T-H05-508194    Patent Document 2: JP-A-H09-310162    Patent Document 3: JP-A-2016-3374